Explain briefly the type of vascular …

There are mainly three types of vascular bundles:

(i) Radial: Those in which the xylem and the phloem lie radically side-by-side (for example, in roots of seed plants). This is the most primitive type.

(ii) Conjoint: Those in which the two types of tissues are separated from one another. Here xylem and phloem together form a bundle. The two sub-types are collateral and bicollateral.

(a) Collateral: The xylem and phloem lie together on the same radius in such a position that xylem lies inwards and the phloem outwards. Here the phloem occurs on one side of the xylem strand. In the dicotyledonous stem, the cambium is found to be present in between xylem and phloem, such bundles are called open (for example, in <i>Helianthus</i>) and when the cambium is absent, it is called closed (for example, in monocotyledonous stems).

(b) Bicollateral: In such bundles, the phloem is found to be present on both sides of xylem. Simultaneously two cambium strips also occur. Various elements are arranged in the following sequence-outer phloem, outer cambium, xylem, inner cambium and inner phloem. Such bundles are commonly found in the members of Cucurbitaceae. Such bundles are always open.

(iii) Concentric: Those in which one type of tissue surrounds or ensheaths the other. The concentric bundles may be of two subtypes, amphivasal bundle as found in Dracaena., Yucca and other monocots and some dicots. If the phloem surrounds the xylem, it is amphicribal as found in many ferns. Such bundles are always closed.